Mesenchymal stem cells (MSCs) have the capacity to develop into a variety of tissue types including bone, cartilage, tendon, muscle, fat and hematopoietic-supportive stroma. Techniques for the isolation of human and animal MSCs have been described as have techniques for directing their differentiation into, inter alia, the osteogenic lineage. These cells have also been shown to retain their developmental potential following extensive subcultivation in vitro (Bruder et al., 1997b), supporting their characterization as stem cells.
Although several monoclonal antibodies (McAb) that react with the surface of human mesenchymal stem cells (hMSCs) in culture have been described (Haynesworth et al., 1992a), the molecular identity of their respective antigens was not addressed. Recently, a new series of McAbs directed against the surface of human MSCs undergoing osteoblastic differentiation was reported (Bruder et al., 1997a). One of these McAbs, referred to as SB-10, reacts with the surface of human MSCs during fetal long bone and calvarial development, as well as the surface of marrow-derived MSCs in culture prior to differentiation. Immunoreactivity is lost during lineage progression when the osteoblast marker alkaline phosphatase (APase) is expressed. The SB-10 antigen has been observed not only in the outer periosteum of developing bones, but also in the developing brain, lung, and esophagus (Bruder et al., 1997a).